A. Technical Field
Fine resolution device and circuits are fabricated by one or a series of steps each involving lithographic resolution followed by selective treatment of regions of device material. Lithography is ordinarily carried out in "actinic" material which is subsequently developed to result in aperture-delineation. Such patterns serve to directly or indirectly mask material undergoing processing. Selective etching may be carried out by dry processing for example, by means of a species produced in a plasma with such procedures being preferred to wet processing where particularly fine resolution is desired.
Such fabrication is used in production of Large Scale Integrated silicon circuitry. Use is contemplated for discrete devices; and in other semiconductor technology; for integrated optical circuitry, for magnetic memories, etc.
B. History
Large Scale Integrated Circuitry as well as other high resolution planar structures are generally fabricated through a series of levels. One procedure, common to construction of most such circuits involves: first producing a masking layer within a continuous region of actinic material by selective exposure to radiation; followed by development to selectively remove material--either radiated or unradiated. Such masking layers have served as or in the fabrication of discrete masks.
This mask technology now in prevalent use in the fabrication of silicon integrated circuits has undergone considerable development to the present point at which design rules of a few microns are regularly attainable. Discrete masks so used serve for secondary delineation of patterns in expendable photoresist layers which serve for actual device processing and which are removed to permit successive processing cycles of similar nature at each fabrication level.
It is generally believed that mask technology will be replaced by a maskless technology (direct processing) at such time that significantly finer resolution is required. In accordance with such comtemplated procedures, primary rather than secondary delineation will be in expendable resist layers tightly adherent to the device undergoing processing.
Regardless of procedures--whether mask or maskless; regardless of involved technology, a procedure common to all such fabrication involves selective etching of continuous layers of device-functional material. To date, wet etching--for example by use of aqueous acid media--has found satisfactory use. As resolution needs become more stringent inherent limitations become more significant. Liquid media reacting with polycrystalline or amorphous layers together result in isotropic etching. Resulting undercutting, to an extent approximately equal to the layer thickness, imposes a limit on spacing.
Increasing miniaturization has resulted in appreciation for advantages for dry processing. Etching by momentum transfer, for example by ion milling, minimizes contamination and imparts directionality to material removal. High accelerating fields with attained particle bombardment of surfaces being processed sometimes causes new problems. Lattice damage at some levels of fabrication may be significant. At the other end of the spectrum, dry processing may depend upon plasma assisted reactions. Plasma etching, for example, is dependent upon removal primarily due to chemical reaction of material to be removed with plasma-produced etching species. As in momentum transfer processes, --reaction product may be inherently removed--in this instance by system selection to result in vapor state product. Plasma assisted etching, however, in the extreme case is again essentially isotropic in behavior. Directionality, imposed for example in reactive ion etching or even by larger permitted energies in plasma etching, while tending toward anisotropic behavior may be undesirable in other respects. Lattice damage and resist erosion are among the problems encountered. As in wet etching, in precise end point detection as well as unequal wafer-to-wafer etching may result not only in extreme undercutting, but also in etching of underlying layers. The latter is alleviated by selection of systems with pronounced selectivity for material being etched.
A variety of materials are encountered in LSI production. They include the many varieties of elemental silicon (polycrystalline, single crystalline, doped, undoped) as well as a variety of silicon containing compounds (oxides as well as nitrides). As in many nonsilicon technologies, the prevalent plasma etchant is that resulting from introduction of mixtures of CF.sub.4 and O.sub.2. Etch rates are generally satisfactory for most materials. undercutting is a major disadvantage as applied to surfaces containing elemental silicon, in fact as applied to most materials to be etched etching behavior is essentially isotropic (lateral etch rate is approximately equal to etch rate in a direction normal to the plane of the surface). Generally good discrimination against a background of silicon compounds (about 10:1 over oxide or nitride) may permit some overetching but does not overcome the profile limitation of feature size/spacing.